In a manner known per se, a turbine engine compressor comprises several compressor stages, each having an annular row of mobile vanes mounted on a rotor shaft and an annular row of variable-pitch stator vanes mounted at their radially external ends on a substantially cylindrical external casing.
Adjustment of the angular setting of the stator vanes in a turbine engine is intended to optimise the output of said turbine engine and reduce its fuel consumption during the different phases of flight.
The variable-pitch stator vanes each comprise at their radially external end a radial pivot, which is centred and rotationally guided in an orifice in the external casing. Each vane pivot is connected by a crank-arm to a control ring that surrounds the external casing of the compressor and is rotationally movable around the longitudinal axis of the compressor by actuating means in order to transmit to the vanes a rotational movement around the axes of their pivots.
Each crank-arm is fixed to the vane pivot and comprises a cylindrical pin inserted in a cylindrical hole in the control ring.
During rotation of the control ring around its axis, the latter causes the crank-arms and the vanes to pivot around the axis of the vane pivot. The total angular range of rotation of the crank-arms is conventionally on the order of 50 to 90°. The ring is also axially movable so as to accompany the path of the pins. All the vanes are in this case situated in the same angular position for a given angular position of the control ring.
Now, depending on the speed of the turbine engine, it is necessary to be able to adapt the pitch of the vanes, particularly as a function of their azimuthal position, i.e. the circumferential position of the stator vane in the corresponding stage. The pitch angles allowing maximisation of the turbine engine output may therefore differ as a function of the azimuthal positions of the stator vanes in a given stage.
Indeed, the gas flow in the stream passing through the high-pressure compressor is not uniform over its entire circumference, wherein said flow may comprise pockets causing losses of performance. Furthermore, when the turbine engine is operating at high speed, high forces and torques are applied to the vanes, which tends to slightly distort the control ring.